LIGHT ACQUISITION AND GROWTH BY COMPETING INDIVIDUALS IN CO2-ENRICHEDATMOSPHERES - CONSEQUENCES FOR SIZE STRUCTURE IN REGENERATING BIRCH STANDS

1 To investigate how CO2-enriched atmospheres may influence plant comp etition and stand size structure in regenerating forests, experimental populations comprised of three maternal families of yellow birch (Bet ula alleghaniensis Britt.) were grown in both ambient (350 mu L L(-1)) and elevated (700 mu L L(-1)) CO2 concentrations in a controlled envi ronment facility. Individual seedling growth, light acquisition, and s tand size structure were monitored throughout the first year of growth . 2 Elevated CO2 increased average seedling biomass in stands by 14%, a value much lower than the average enhancement reported elsewhere for individually grown yellow birch seedlings. Maternal families within s tands differed significantly in their growth responsiveness to elevate d CO2, ranging from +51% to -16%. As a result, CO2 altered the genetic identity of dominants in regenerating stands. 3 Seedling size inequal ities were generally lower in CO2-enriched environments, a result that contrasts with other studies that have reported increased size inequa lity with increased productivity in resource-rich environments. Distri bution modifying functions relating initial seedling size and subseque nt growth suggest that there was a relatively smaller advantage to bei ng larger in elevated vs. ambient CO2 environments. Together, these re sults suggest that competition in CO2-enriched environments was less s ize-asymmetric. 4 Differences in stand size structure between CO2 trea tments were related to competition for light. Empirical measures of se edling light acquisition per unit biomass suggest competition for ligh t was less size-asymmetric in CO2-enriched environments. Decreased siz e-asymmetric competition for light was attributable both to difference s in the CO2-use efficiency of high-light canopy dominants vs. low-lig ht canopy subordinates, and to CO2-induced differences in plant allome try. 5 This study highlights the importance of stand-level competition studies in global change research, and more generally, the value of s tudies that combine phenomenological descriptions of stand development with physiological mechanisms of competition.